In recent years, plant growth using an artificial light source has been studied. In particular, attention is being paid to a cultivation method using lighting by a light-emitting diode (LED) which has excellent monochromaticity and in which energy saving, long service life, and a reduction in size are possible. Further, from previous study results, as one of luminescence wavelengths suitable for a light source for plant growth (photosynthesis), the effects of red light having a wavelength range of 600 nm to 700 nm have been confirmed. Further, there is also a report that infrared light in a range of 730 nm to 760 nm is also an effective wavelength for cultivation control. In particular, with respect to photosynthesis, light having a wavelength of around 660 nm to 670 nm is the most desirable light source having high reaction efficiency. With respect to these wavelengths, in an existing light-emitting diode, a light-emitting layer made of AlGaAs on a GaAs substrate has been put to practical use (for example, Patent Documents 1 to 3).
On the other hand, a compound semiconductor LED provided with a light-emitting layer made of aluminum gallium indium phosphide (composition formula: (AlXGa1-X)YIn1-YP (0≦X≦1 and 0<Y≦1)) is known. In this LED, the peak wavelength which is obtained by yellow-green to red light-emitting layers is around 560 nm to 670 nm.
Further, a light-emitting section provided with the light-emitting layer made of (AlXGa1-X)YIn1-YP (0≦X≦1 and 0<Y≦1) generally has been formed on a gallium arsenide (GaAs) single crystal substrate. There is a problem in that the gallium arsenide (GaAs) single crystal substrate is optically opaque to luminescence which is emitted from the light-emitting layer and is not very strong mechanically. Therefore, in order to obtain a visible LED having higher brightness, research aimed at improvement in mechanical strength of an additional element has been carried out. That is, a technique of constituting a so-called junction type LED has been disclosed in which after an opaque substrate material such as GaAs is removed, a support layer made of a transparent material capable of transmitting luminescence and also having superior mechanical strength than that in the past is joined again (refer to Patent Document 4, for example).
Further, with respect to a high-performance protective film for providing countermeasures against high humidity of a light-emitting diode, a two-layer structure using an inorganic film and a thermosetting resin film has been proposed (refer to Patent Document 5, for example).
Further, with respect to a high-functioning protective film of an AlGaAs-based light-emitting diode, a method of forming a protective film having a sufficient film thickness greater than 100 nm on a semiconductor layer has been proposed (refer to Patent Document 6, for example).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 9-37648    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-27831    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-221042    [Patent Document 4] Japanese Patent No. 3230638    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 8-298341    [Patent Document 6] Japanese Unexamined Patent Application Publication No. 7-94777